1. Field of the Invention
The present invention relates to a grating as a wavelength separation/selection element used in a spectroscope or a branching filter, and negative and replica gratings manufactured by transfer from the grating and methods of manufacturing these gratings.
2. Description of the Related Art
A grating is a wavelength separation/selection element used in a spectroscope, a branching filter, etc. The known gratings are roughly classified according to the groove cross-section shape into (1) a holographic grating, (2) a brazed holographic grating or a ruled grating, and (3) a laminar grating. The holographic grating is manufactured by exposing and developing interferential fringes formed by two luminous flux interference (holographic exposure method) onto a photoresist layer coated on a substrate. The holographic grating has a resist pattern with the groove cross section shaped like a sinusoidal wave. The grating with the groove cross section shaped like the sinusoidal wave will be hereinafter referred to as holographic grating. The brazed holographic grating is manufactured by converting the groove cross-section shape of the (1) holographic grating into a sawtooth shape by an ion beam machining technique. The ruled grating has the groove cross section shaped like a sawtooth, ruled by a ruling engine, etc. The gratings each with the groove cross section shaped like the sawtooth will be hereinafter referred to collectively as echellette gratings. The laminar grating is manufactured by converting the groove cross-section shape of the (1) holographic grating into a rectangle shape by the ion beam machining technique.
Light is a transverse wave having two components of an electric wave and a magnetic wave which run at right angles to one another. Essentially the action in the boundary region between the electric wave and the magnetic wave varies. Therefore, to obtain the diffraction efficiency of the grating, it is necessary to separate the light incident on the grating into a component vibrating in parallel to the groove direction and a component vibrating perpendicular to the groove direction, and calculate the action on the grating groove surface for each of the components. However, if the used wavelength in light is small relative to the groove cycle of the grating, without discussion with the incident light separated into the two components, only the strength of light, namely, Fraunhofer diffraction of each grating groove is integrated with every grating groove, whereby the diffraction efficiency is simply calculated and a good match with the actual one can be provided. This calculation theory is called scalar theory. According to the theory, a good calculation result well matching the actual one can be provided.
A region of groove cycle/wavelength>5 (the groove cycle relative to the wavelength being more than 5) is called scalar domain. Fraunhofer diffraction to which each grating groove contributes is integrated, whereby the diffraction efficiency can be calculated. In the scalar domain, the difference caused by polarization in the spectrum shape is small.
In contrast, a region of groove cycle/wavelength<5 (the groove cycle relative to the wavelength being less than 5) is called resonance domain and the scalar theory does not hold. In the resonance domain, the action in the boundary region depending on polarization varies. Thus, to obtain the diffraction efficiency, the action on the grating groove surface needs to be strictly calculated with the light incident on the grating as vector quantity.
Generally, for spectrum of a short wavelength such as radiation, the laminar grating with the groove cross section shaped like the rectangle is often used. For a spectroscope of an analyzer of wavelengths from ultraviolet to near infrared, the echellette grating with the groove cross section shaped like the sawtooth is mainly used. Although there are various reasons, a groove shape with the optimum diffraction efficiency mainly depends on the used wavelength zone, the grating use method, etc. In the scalar domain, the calculation result based on the scalar theory matches the actual one comparatively well. However, in the resonance domain wherein the groove cycle and the wavelength become the same degree for wavelengths from near infrared to infrared, the holographic grating with the groove cross section shaped like the sinusoidal wave is often used because it may be excellent in diffraction efficiency more than the echellette grating.
Replica gratings are generally mass-produced as follows: A thin oil film or a metal film which has a weak adhesion force such as gold or platinum, as a release agent is formed on a grating face of a negative grating. An aluminum thin film is formed thereon by vacuum evaporation. Then a replica substrate (glass substrate) is bonded onto the aluminum thin film with an adhesive. After the adhesive is hardened, the glass substrate is parted from a master block (negative grating). The aluminum thin film is parted from the master block together with the glass substrate. Consequently, the replica grating to which the grating grooves of the negative grating are transferred can be provided.
When the grating is manufactured, if an attempt is made to provide a resolution in the used wavelength zone, it is necessary to increase the number of grooves of the grating (lessen the groove cycle). When the groove cycle and the used wavelength become the same degree, the groove depth relative to the groove cycle (aspect ratio) needs to be made large. However, as for the holographic grating, it is difficult to produce good-contrast interferential fringes stably during exposure because the groove cross section of the interferential fringe at the exposure is shaped like the sinusoidal wave, and there are disturbance of vibration, heat, etc., at the exposure. Thus, a resist pattern with a deep groove depth cannot be formed. Consequently, holographic gratings or brazed holographic gratings each with a deep groove depth cannot be manufactured.
When a holographic grating or an echellette grating with a reasonably large aspect ratio in the range in which the grating can be manufactured is replicated, a release agent is not effectively put on the groove surface and breakage easily occurs at the parting stage. For example, at the parting, grating grooves are chipped or the groove shape is not faithfully transferred and thus the performance of the grating is degraded, namely, often the manufacturing efficiency worsens extremely. To avoid such a problem in the parting stage, a small aspect ratio and an easy-to-part shape are desired. However, to attempt to obtain a high resolution for a grating used particularly in wavelengths from near infrared to infrared, the aspect ratio becomes large, and the absolute groove depth also becomes deep; the above-described problem is more conspicuous.
Further, for the holographic grating and the echellette grating, in the resonance domain, even if the aspect ratio is made large, the peak remains on the short wavelength side. Thus, it is also difficult in theory to bring the peak into any desired wavelength. Therefore, it is difficult to form a reflection (transmission) band having sufficient diffraction efficiency in a nearby wavelength zone for the used wavelength.